[Python] RuntimeError: The current Flask app is not registered with this 'SQLAlchemy' instance

I'm having troubles to init my app on flask. It says "Did you forget to call 'init_app', or did you create multiple 'SQLAlchemy' instances?" I call init_app as "db.init_app(app)" and as far as I look to the code I avoid to create multiple SQLAlchemy instances. I call an extensions.py just to create a SQLAlchemy instance and call it on models and on main, I don't create this instance anywhere else. here is the main.py:

# main.py
import os
from flask import Flask, render_template, url_for, flash, redirect, request, Blueprint, jsonify, abort, current_app
from werkzeug.utils import secure_filename
from PIL import Image
from datetime import datetime
from config import Config
from .extensions import db, login_manager
from .models import Produto, DadosFormulario, User # Import User model
from .forms import ProductForm # Assuming you have a forms.py with ProductForm
from flask_login import login_required, current_user, login_user, logout_user # Add login_user, logout_user

# Define the Blueprint
main = Blueprint('main', __name__)

# --- Authentication Routes (Add these if you don't have them yet) ---
@main.route('/login', methods=['GET', 'POST'])
def login():
if current_user.is_authenticated:
return redirect(url_for('main.profile'))
# Assuming you have a LoginForm in forms.py
from .forms import LoginForm # Import LoginForm here
form = LoginForm()
if form.validate_on_submit():
user = User.query.filter_by(email=form.email.data).first()
if user and user.password == form.password.data: # In a real app, use werkzeug.security.check_password_hash
login_user(user, remember=form.remember.data)
next_page = request.args.get('next')
return redirect(next_page) if next_page else redirect(url_for('main.profile'))
else:
flash('Login Unsuccessful. Please check email and password', 'danger')
return render_template('login.html', title='Login', form=form)

@main.route('/logout')
def logout():
logout_user()
return redirect(url_for('main.index'))

# --- Existing Routes ---
@main.route('/')
def index():
return render_template('index.html')

@main.route('/profile')
@login_required
def profile():
return render_template('profile.html', name=current_user.name)

@main.route('/contato')
def contato():
return render_template('contato.html')

@main.route('/enviar', methods=['POST'])
def enviar():
nome = request.form['nome']
email = request.form['email']
estado = request.form.get('estado')
cidade = request.form.get('cidade')
mensagem = request.form['mensagem']

novo_dado = DadosFormulario(nome=nome, email=email, estado=estado, cidade=cidade, mensagem=mensagem)
db.session.add(novo_dado)
db.session.commit()
flash('Sua mensagem foi enviada com sucesso!', 'success')
return redirect(url_for('main.contato'))

@main.route("/mostrar_produtos", methods=["GET"])
def mostrar_produtos():
produtos = Produto.query.all()
return jsonify([produto.to_json() for produto in produtos])

@main.route("/produto/<int:id>", methods=["GET"])
def mostrar_produto(id):
produto = Produto.query.get(id)
if produto is None:
abort(404)
return jsonify(produto.to_json())

# --- Functions ---
def allowed_file(filename): # Removed app_config parameter
return '.' in filename and \
filename.rsplit('.', 1)[1].lower() in current_app.config['ALLOWED_EXTENSIONS']

def save_picture(form_picture): # Removed app_instance parameter
random_hex = os.urandom(8).hex()
_, f_ext = os.path.splitext(form_picture.filename)
picture_fn = random_hex + f_ext
picture_path = os.path.join(current_app.root_path, current_app.config['UPLOAD_FOLDER'], picture_fn)

output_size = (125, 125)
i = Image.open(form_picture)
i.thumbnail(output_size)
i.save(picture_path)

return picture_fn

# --- Product Routes ---
@main.route('/product/new', methods=['GET', 'POST'])
@login_required
def add_product():
form = ProductForm()
if form.validate_on_submit():
picture_file = 'default.jpg'
if form.image.data:
picture_file = save_picture(form.image.data) # No need to pass current_app

product = Produto(name=form.name.data, price=form.price.data,
description=form.description.data, image_file=picture_file)
db.session.add(product)
db.session.commit()
flash('Produto adicionado com sucesso!', 'success')
return redirect(url_for('main.index'))
return render_template('add_product.html', title='Adicionar Produto', form=form)

@main.route('/product/<introduct_id>')
def product(product_id):
product = Produto.query.get_or_404(product_id)
return render_template('product.html', title=product.name, product=product)

@login_required
@main.route('/product/<introduct_id>/edit', methods=['GET', 'POST'])
def edit_product(product_id):
product = Produto.query.get_or_404(product_id)
form = ProductForm()
if form.validate_on_submit():
if form.image.data:
if product.image_file != 'default.jpg':
old_image_path = os.path.join(current_app.root_path, current_app.config['UPLOAD_FOLDER'], product.image_file)
if os.path.exists(old_image_path):
os.remove(old_image_path)
picture_file = save_picture(form.image.data)
product.image_file = picture_file
product.name = form.name.data
product.price = form.price.data
product.description = form.description.data
db.session.commit()
flash('Produto atualizado com sucesso!', 'success')
return redirect(url_for('main.product', product_id=product.id))
elif request.method == 'GET':
form.name.data = product.name
form.price.data = product.price
form.description.data = product.description
return render_template('edit_product.html', title='Editar Produto', form=form, product=product)

@login_required
@main.route('/product/<introduct_id>/delete', methods=['POST'])
def delete_product(product_id):
product = db.session.get(Produto, product_id)
if product is None:
abort(404)

if product.image_file != 'default.jpg':
image_path = os.path.join(current_app.root_path, current_app.config['UPLOAD_FOLDER'], product.image_file)
if os.path.exists(image_path):
os.remove(image_path)

db.session.delete(product)
db.session.commit()
flash('Produto excluído com sucesso!', 'success')
return redirect(url_for('main.index'))

# --- Application Factory ---
def create_app():
app = Flask(__name__)
app.config.from_object(Config)

# Initialize extensions with the app
db.init_app(app)


# Register Blueprints
app.register_blueprint(main)

return app

if __name__ == '__main__':
app = create_app()
with app.app_context():
db.create_all()
app.run(debug=True)
`````
here is the models.py:
````
# models.py
from datetime import datetime
from .extensions import db
from flask_login import UserMixin # Keep UserMixin import



class User(UserMixin, db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(100), unique=True, nullable=False)
email = db.Column(db.String(100), unique=True, nullable=False)
password = db.Column(db.String(200), nullable=False)

def __repr__(self):
return f"User('{self.name}', '{self.email}')"



class Produto(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(100), nullable=False)
price = db.Column(db.Float, nullable=False)
description = db.Column(db.Text, nullable=False)
image_file = db.Column(db.String(20), nullable=False, default='default.jpg')

def to_json(self):
return {
'id': self.id,
'name': self.name,
'price': self.price,
'description': self.description,
'image_file': self.image_file
}

class DadosFormulario(db.Model):
id = db.Column(db.Integer, primary_key=True)
nome = db.Column(db.String(100), nullable=False)
email = db.Column(db.String(100), nullable=False)
estado = db.Column(db.String(100), nullable=True)
cidade = db.Column(db.String(100), nullable=True)
mensagem = db.Column(db.Text, nullable=False)
data_envio = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
````
and extensions.py:
````
from flask_sqlalchemy import SQLAlchemy
from flask_login import LoginManager
login_manager = LoginManager() # Initialize LoginManager here as well
db = SQLAlchemy()
`````

Continue reading...